1. Field of the Invention
The present invention relates to a band pass filter in which a predetermined pass band is set and to a filter module including such a filter.
2. Description of the Related Art
In recent years, multi-mode- and multi-band-compliant communication terminals have been provided. Such communication terminals include a band pass filter that allows only a desired communication signal to pass therethrough. For example, as illustrated in FIG. 11, a band pass filter 500 formed by connecting a concentrated-constant LC high pass filter 501 and a concentrated-constant LC low pass filter 502 in series has been proposed (see Japanese Unexamined Patent Application Publication No. 9-181549 (paragraphs [0008] and [0009], FIGS. 1 and 2, abstract, etc.), for example). Specifically, the LC high pass filter 501 is constituted of two capacitors C51 and C52 connected in series in a path W51 connecting an input terminal A51 to an output terminal B51, and an inductor L51 connected between a ground potential and a path between the capacitor C51 and the capacitor C52. Meanwhile, the LC low pass filter 502 is constituted of a single inductor L52 connected in series in the path W51, and two capacitors C55 and C56, each connected between the ground potential and the path W51 at respective ends of the inductor L52.
The band pass filter 500 is formed by the LC high pass filter 501 and the LC low pass filter 502 being connected in series and a predetermined pass band being set. This makes it possible to achieve a wider band for the pass band of the band pass filter 500. Furthermore, circuit constants of the band pass filter 500 can be adjusted by adding a capacitor C53 in series to the inductor L51 of the LC high pass filter 501 and adding a capacitor C54 in parallel to the inductor L52 of the LC low pass filter 502, which makes it possible to set the pass band characteristics of the band pass filter 500 as desired.
In multi-mode- and multi-band-compliant communication terminals, communication is carried out by selectively using a plurality of communication signals, each having a different frequency band. There is a recent trend toward the frequency bands assigned to the respective communication signals being close to each other, and thus there is demand for a band pass filter having steeper attenuation characteristics.